1. Field of the Invention
The present invention relates to a discharge lamp especially suited for a forward lighting application of an optical apparatus and more particularly, to a metal halide discharge lamp operating from a D.C. power source suited for an optical light source of an projector device such as a projection color display apparatus.
2. Description of the Related Art
Recently, it has been proposed to utilize a metal halide discharge lamp as a light source for a projection color display apparatus. U.S. Pat. No. 5,135,300 discloses a projection color display apparatus in which a metal halide lamp as a white light source is assembled. The light source uses a parabolic reflecting mirror having a concave surface so as to converge the light flux emitted from the light source with efficiency and to obtain parallel rays. The light source is made as close to a point source as possible and it is mounted within the reflecting mirror so as to coincide the focal point of the reflecting mirror with the center of the light output from the light source. The metal halide discharge lamp has a pair of electrodes, and the gap length between the electrodes is shorter than conventional metal halide discharge lamps used for general illumination purpose such as street lights and flood lights. The discharge lamp disclosed in the patent is called a short arc metal halide discharge lamp.
As for metal halide additives filled in an arc tube of a metal halide discharge lamp, dysprosium iodide, indium iodide, thallium iodide and cesium iodide are known to efficiently produce red, green and blue components of the light. In operation of such a metal iodide discharge lamp, all of mercury is vaporized, resulting in a high-pressure, wall-stabilized arc in a gas consisting principally of mercury vapor at several atomospheres pressure. The iodides also evaporate from the arc tube walls, the iodine molecules diffusing into the high-temperature arc column, where they dissociate. The metal atoms are ionized and excited and give off their own characteristic spectral lines. As metal atoms diffuse back to the walls, they encounter iodine atoms in the cooler gas near the walls and recombine to reform the iodine molecules.
When a short arc metal halide discharge lamp is operated, the power consumption of the arc column or the wall load of the lamp defined the power consumed by the arc column pep inside wall surfaces of the arc tube (W/cm.sup.2) becomes extremely high to raise the wall temperature of the arc tube almost to its melting point, and metal ions of the additives, such as metal ions of rare-earth metals or metal ions of alkali metals react with silica (SiO.sub.2) of the arc tube considerably to lose the transparency of the arc tube. It is assumed that silica at the high temperature tube walls exposed by the metal ions tends to be transformed its crystalline structure to a cristobalite known as an opaque substance easily. Such transformation firstly occurs at the hottest wall areas after 100 hours operation. Loss of the transparency spreads over the other wall areas and the whole wall areas are finally covered with the opaque substance. After 1000 hour-operation, the inside walls of the arc tube is observed as if white refractory materials are deposited thereon, and the transparency of the arc tube is almost lost. A metal halide discharge lamp having such a non-transparent arc tube is no longer used for a point light source because the arc tube diffuses the light emitted from by the arc column.
In a metal halide lamp operated from a direct current (D.C.) power source, it typically experiences the effects of catephoresis which cause the halides of the metal halide discharge lamp to be moved or swept into the end regions of the lamp so as not to contribute to providing the desired illumination of such lamp. Conventionally, it has been tried to reduce the detrimental effects of catephoresis as disclosed in U.S. Pat. No. 4,935,668.
It is an object of the present invention to provide an improved metal halide discharge lamp operating from a D.C. power source, which is suitable for a point light source.
It is another object of the present invention to provide a metal halide discharge lamp operating from a D.C. power source which utilizes the catephoresis effects to prevent loss of transparency of an arc tube of the lamp.
It is further object of the present invention to provide a metal halide discharge lamp operating from a D.C. power source which has a high luminous efficacy and long operating life.
It is still further object of the present invention to provide a color display device in which an optical light source mounting a metal halide discharge lamp operating from a D.C. power source.